This invention relates generally to a load extension axle assembly adapted for use in combination with a conventional tractor and trailer to increase load capacity and to extend the road length over which the load is distributed. In particular, such an assembly, which may alternatively be referred to as a tag axle assembly, is useful to increase and extend the loads carried by tandem axle logging trailers.
Tractor-trailers which haul large loads over highways and bridges must conform to restrictions relating to vehicle weight and load distribution. These restrictions typically are specified in terms of axle loads, number of axles and distribution of the load among axles, groups of axles, and over a given length of roadway, and are commonly known as "bridge formulas." Tractor-trailers adapted for carrying logs typically have a truck-tractor and a trailer connected together by a compensating reach. A first bunk is pivotally mounted by means of a fifth wheel assembly over the drive axles of the truck-tractor to support a forward end of the load. A second bunk is conventionally mounted on the trailer chassis, by means of a fifth wheel assembly mounted on a tandem axle. The load itself transmits towing forces from the truck-tractor to the trailer. The trailer is hitched to the truck-tractor by a reach which is freely extensible for compensating in length while steering the trailer tandem axle. Being mounted on fifth wheel assemblies or the like, the bunks can swivel about a vertical axis while the vehicle is turning. Heretofore, a number of arrangements have been proposed to increase the load carrying capacity, and extend the lengthwise distribution of load, of such vehicles.
U.S. Pat. No. 4,219,210 to Genberg exemplifies the current state-of-the-art in load extenders for logging trucks. This patent shows a tractor-trailer set up for carrying logs substantially as described above. An auxiliary trailer is hitched to the log trailer to increase its capacity and extend its load over an increased length of road surface or bridge span. The auxiliary trailer has a tongue extending forwardly from an auxiliary rear axle and coupled to the main trailer chassis over the dual axles in the location conventionally occupied by the rear bunk. The forward end of the auxiliary tongue includes a trunnion assembly which is mounted atop the fifth wheel or turntable assembly, in place of the rear bunk. The rear bunk is shifted rearwardly, to a position aft of the trailer's tandem axle, and is mounted on a fifth wheel or turntable assembly on the auxiliary trailer tongue.
This arrangement permits the auxiliary trailer to pivot about vertical and horizontal axes which intersect in a common plain with the rocking axis of the tandem axles of the trailer. The trunnion allows the auxiliary axle to rise and fall with variations in terrain and the fifth wheel allows the auxiliary trailer to swing laterally behind the log trailer for turning.
A drawback of this design is that, in turns, the rear portion of the load shifts out of alignment with the central axis of the tandem axles of the trailer. This combination requires a wider swept path and more maneuvering area in turns, as acknowledged by Genberg at column 7, lines 58-68. This arrangement also affects load stability, which can further limit the speed and minimum radius at which turns can be made safely. A commercial load extender made by Cascade Trailer, Inc. of Portland, Oreg., resembles the Genberg load extender but requires customization of the pull trailer chassis. Log trailers using it have turned over while turning when loaded with logs.
Another drawback, acknowledged by Genberg at column 8, lines 50-56, is the difficulty of reversing or backing up with the disclosed auxiliary trailer assembly. The auxiliary tongue of Genberg is also fixed in length. This gives a fixed load distribution among the trailer axles and auxiliary axle and is disadvantageous in off-road terrain.
A number of other structures have been proposed to increase load capacity and distribution. U.S. Pat. No. 4,966,383 to Sjostrom discloses a forward, self steering axle which is mounted forward of the tandem axles and bunk of a conventional log trailer. A portion of the load is transferred to the forward axle by means of leaf springs. U.S. Pat. No. 3,685,853 to Goldsmith and U.S. Pat. No. 3,910,601 to Michaud each disclose a trailing rear axle which is arranged to be raised and lowered on a pivot arm by means of a hydraulic ram, to alter the proportion of load transferred to the auxiliary axle. In Goldsmith, the pivot and trunnion axes intersect as in Genberg. In Michaud, the horizontal trunnion axis is located rearward of the vertical pivot axis.
U.S. Pat. No. 2,919,928 to Hoffer discloses a drop-deck type of trailer in which load is supported between two sets of tandem axles. This trailer has a bogey assembly with caster wheels. The main rocking beam of the bogey assembly is connected to the rear end of the trailer by means of a trunnion positioned rearward of the trailer axles. A lock shaft anchors the front end of the main beam at a point centered between the tandem axles of the trailer. The auxiliary trailer axles themselves are mounted in castor fashion so as to trail behind a vertical pivot axis.
U.S. Pat. No. 3,082,039 discloses a conventional tag trailer which supports an independent load on an auxiliary axle. U.S. Pat. No. 3,450,222 to Ohlson discloses a self propelled vehicle having three wheeled sections wherein a portion of the load is transferred to the rearmost axle by means of a rigid lengthwise connection.
None of the foregoing patents address the drawbacks noted above in connection with the Genberg and Cascade Trailer load extenders. Several of these designs show some ability to extend or retract the load extension beam or tongue. Prior extension/retraction systems nonetheless are complex and expensive, inconvenient to operate, and/or would be impractical to integrate into an auxiliary trailer assembly for logging trailers, such as that of Genberg.
Accordingly, a need remains for a better arrangement for increasing load capacity and extending the lengthwise distribution of load in tractor-trailer combinations, particularly those used for hauling logs and the like.